3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB.
FIG. 1 illustrates a conventional radio access network in an LTE system. An eNodeB 101a with a transmission point 102a serves a UE 103 located within the eNodeB's geographical area of service also called a cell 105a. The eNodeB 101a is directly connected to the core network (not illustrated). The eNodeB 101a is also connected via an X2 interface to a neighboring eNodeB 101b with a transmission point 102b serving another cell 105b. 
The use of a so called heterogeneous deployment or heterogeneous network consisting of radio network nodes transmitting with different transmit power and operating within overlapping coverage areas, is an interesting deployment strategy for cellular networks. In such a deployment schematically illustrated in FIG. 2a, low-power nodes such as pico nodes 210 are typically assumed to offer high data rates measured in Mbit/s, as well as to provide high capacity e.g. measured in users/m2 or in Mbit/s/m2, in the local areas where this is needed or desired. High-power nodes, often referred to as macro nodes 220, are assumed to provide full-area coverage. Pico nodes and macro nodes may also be referred to as pico RBSs and macro RBSs respectively.
In a traditional heterogeneous deployment, schematically illustrated in FIG. 2b, a macro node 220 creates a macro cell 221 and each pico node 210 creates a cell of its own, a so called pico cell 211. This means that, in addition to downlink and uplink data transmission and reception on the pico link 213 maintained between the pico node 210 and the wireless device 212, the pico node 210 also transmits the full set of common signals and channels associated with a cell. In an LTE context this includes the primary and secondary synchronization signals, Cell-specific Reference Signals (CRS), and system information (SI) related to the cell, in FIG. 2b referred to as SI pico and illustrated by a cell with a dashed line overlying the pico cell 211.
In an alternative to the deployment illustrated in FIG. 2b, a terminal or wireless device 212 in the range of a pico node 210, i.e. in the subarea 214 covered by the pico node, may be simultaneously connected to both a macro node 220 and the pico node 210 as illustrated in FIG. 2c. To the macro node 220, covering the area 222, the terminal 212 maintains a connection or link, e.g. used for radio-resource control (RRC) such as mobility. Furthermore, the terminal 212 maintains a connection or link to the pico node 210, used primarily for data transmission. This approach may be referred to as a combined cell or soft cell approach. In the following it will be referred to as the combined cell approach. The SI related to the combined cell is in FIG. 2c referred to as SI and is illustrated by a cell with a dashed line overlying the area 222.
The distinction between cell and transmission points is an important aspect of the combined cell approach. Each cell has a unique cell identity from which the CRS is derived. With the cell identity information, a terminal can derive the CRS structure of the cell and obtain the SI it needs to access the network. A transmission point on the other hand is mainly one or more collocated antennas from which a terminal can receive data transmissions in a certain area. As a conclusion, a cell may be deployed with one or several antennas or transmission points covering the cell area. In the latter case, the cell is thus served by a plurality of transmission points where each transmission point covers an area of the cell, hereinafter also referred to as a sector of the cell.
Mobile user positioning is the process of determining UE coordinates in space. Once the coordinates are available, the position can be mapped to a certain place or location. There exist a variety of positioning techniques in wireless communications networks, differing in their accuracy, implementation cost, complexity, and applicability in different environments.
Observed Time Difference of Arrival (OTDOA) is a positioning method defined by 3GPP for LTE which exploits a multi-lateration technique to calculate the UE position based on Time Difference of Arrival (TDOA) measurements from three or more locations. To enable positioning, the UE should thus be able to detect signals from at least three geographically dispersed RBSs. This implies that the signals need to have high enough signal-to-interference ratios. Furthermore, the signals need to be transmitted frequently enough to meet the service delay requirements.
OTDOA positioning is using Reference Signal Time Difference (RSTD) measurements as specified in the 3GPP standard i.e. the relative timing difference between the timing of a neighbour cell and a reference cell. It has been shown that using synchronization signals (SS) and CRS for positioning without interference management, results in positioning coverage problems due to low SINR and/or insufficient number of strong signals from different RBS. To address these issues and enhance positioning measurements, Positioning Reference Signals (PRS) have been introduced. PRS are transmitted in downlink according to a predefined PRS pattern. This implies that the UE needs to know what PRS pattern that is used for a certain cell, in order for it to be able to detect the PRS and perform the positioning measurements needed for the positioning services.
The principle of the OTDOA positioning method in a conventional E-UTRAN such as the one illustrated in FIG. 1, is schematically illustrated in FIG. 3a. The cells 305a-c has one single transmission (Tx) point each 302a-c from which the PRS is sent. The Tx points 302a-c are equivalent to the transmission antenna(s). For the case where there are multiple antennas for one Tx point, the spatial separation is still limited and the antennas can be defined by one point. The position of a UE 303 is estimated by measuring RSTD of the different PRS sent from the different surrounding cells' Tx points 302a-c respectively. The timing of the PRS transmissions are synchronized to a common time source. With knowledge of the transmission time, the Tx point location and the measured RSTDs, a positioning function can estimate the position of the UE 303 by hyperbolic trilateration.
However, when moving to a cell deployment that has multiple Tx points within one cell, such as in the combined cell deployment in FIG. 2c, the OTDOA method will not work as wanted. The reason is that the OTDOA positioning method assumes a single Tx point per cell. The Tx points must be spatially unique as ambiguities will otherwise occur in the trilateration calculations, as illustrated in FIG. 3b. Two of the cells 305a-b have one Tx point each 302a-b. However, one of the cells 305c has two geographically separated Tx points 302c and 302d. As these two Tx points 302c-d belong to the same cell 305c, they transmit the same PRS. The UE 303 will therefore measure two different RSTD for the cell 305c which will cause an ambiguous positioning estimate.